zavdi@dduck.ctt.bellcore.com (Zavdi Lichtman) (12/29/89)
A 512K Mac (upgraded from 128K) "tics", i.e. tic, tic, tic ... like a clock, when power is turned on, but nothing appears on the screen- not even the insert diskette flashing icon. The battery is ok. I'll appreciate any advice on what to check/replace. Thanks- zavdi ps. This is an important mac, it is used by my kids.
oster@dewey.soe.berkeley.edu (David Phillip Oster) (12/30/89)
That ticking means the circuit breaker in the power-supply is continually resetting itself. This usually means something is shorted out inside.
gordon@cs.washington.edu (Gordon Davisson) (12/30/89)
In article <18701@bellcore.bellcore.com> zavdi@dduck.ctt.bellcore.com (Zavdi Lichtman) writes: >A 512K Mac (upgraded from 128K) "tics", i.e. tic, tic, tic ... like a clock, >when power is turned on, but nothing appears on the screen- not even the >insert diskette flashing icon. The battery is ok. >I'll appreciate any advice on what to check/replace. Sounds like the power supply is shutting down due to incorrect loading (and then restarting, and then shutting down, and ...). The most common causes of this are a pair of diodes (CR1 and CR5) in the video circuit that tend to fail as short circuits. They can be tested in situ, but it's a bit chancy. You're better off removing them first (be sure to remember which way around they go). CR1 is the one at the top of the analog board near the back, and CR5 is the one attached to the big heat sink just forward of the flyback transformer. Use an ohmmeter to see if either of them conducts in both directions. If one does, you've found the problem. You may have to look around a bit for a replacement -- call local electronic supply stores and ask for GI854 diodes (or GI824, a higher-current version, or MR854 or MR825, the Motorola equivalents, or...). If all else fails, you can send it to me (see .sig). I do component- level repair, which doesn't cost nearly as much as an Apple board-swap. -- Gordon Davisson Westwind Computing (206) 632-8141 4518 University Way NE, Suite 313, Seattle WA 98105